1. Field of the Invention
The present invention relates to a composite video signal generator for use in an on-screen display apparatus which deals with composite video signals and displays predetermined characters on a television screen.
2. Description of the Related Art
FIG. 1 is a block diagram exemplifying a conventional composite video signal generator, and FIG. 2 is a timing chart showing its operation.
Conventionally, this type of a composite video signal generator has a modulator 1, which generates a modulated signal a with its phase modulated to that of a color subcarrier corresponding to each color, and a color signal generator 2, which alternately selects high DC voltages 17 and 18 or low DC voltages 19 and 20 that correspond to individual colors, thereby forming the waveform of each color signal f. A color selector 3 of the composite video signal generator selectively outputs a color signal e according to a red signal 13, a green signal 14 or a blue signal 15, all received as character signals. An output selector 4 consisting of a switch 23 selects the color signal e or a composite video input signal 11 in accordance with a character data presence/absence signal 12 to output a composite video output signal 16 which has character data combined therein.
This conventional composite video signal generator completely switches all the color signals from one to another in accordance with the red, green and blue signals as well as the character data presence/absence signal. The color signal is therefore switched with the maximum amplitude at the change point between color signals or at the change point between each color signal and the composite video input signal. This results incomplete separation of the composite video output signal into a luminance component and a color component in a television or the like, so that a dot disturbance or a color mixture will occur due to mixing of the luminance component into the color component.